In many applications for power MOSFETs, it is important to accurately determine the current being carried by the MOSFET. For example, when power MOSFETs supply current to the headlights of an automobile, it is necessary to know how much current the headlights are drawing to determine if there is a malfunction.
In one example configuration of power MOSFETs, two or more power MOSFETs are on a single integrated circuit die formed on a semiconductor substrate. In this configuration, a portion of the semiconductor substrate serves as the drain contact. One configuration of this type of vertical MOSFET is the NexFET™ devices available from Texas Instruments Incorporated, the assignee of this application. To provide the necessary current capacity, multiple MOSFETs are in a channel and are connected in parallel. To detect the current carried in the MOSFET channel, one smaller MOSFET, a pilot field effect transistor (FET) or sense FET is connected to the power MOSFETs in a current mirror configuration. A current sense output coupled to the current flowing in the pilot FET receives current proportional to the current flowing in the power MOSFETs. The sensed current is available for use by the system. However, when one MOSFET channel in a multichannel power FET integrated circuit is drawing a great deal of current, the current for that channel may also flow through a portion of the common drain used by the other MOSFET channels. This is referred to as “current spreading.” Current spreading in a shared drain MOSFET can lead to inaccurate current measurement results, among other problems.